The present invention concerns a clamping device for securing a draw-in bolt on a machine table, a baseplate, or the like, having a piston that is arranged movably in a cylinder bore to form a pressure chamber, a cover that closes off the cylinder bore on the side opposite the pressure chamber, a receptacle which is configured in the piston and/or the cover and into which a draw-in bolt can be inserted, and a clamping mechanism in order to secure a draw-in bolt in the receptacle, the clamping mechanism being actuable by displacement of the piston between a clamped position into which it is pushed by spring elements that are arranged in a spring chamber provided between the piston and cover, and a released position into which it can be brought by impingement of a hydraulic medium on the pressure chamber.
Clamping devices of this kind operating in mechanical/hydraulic fashion are known, for example, from DE 298 04 730 U1, and are used in practice to secure workpieces to the machine table of a machine tool. For this purpose, the workpiece that is to be machined is usually first clamped onto a carrier plate or carrier pallet; and then the entire unit comprising the carrier pallet and workpiece is clamped in place on the machine tool, by way of several draw-in bolts that are provided on the back side of the carrier pallet and are configured as centering elements, by inserting the draw-in bolts into the receptacles of the clamping devices provided on the machine table and clamping them in place therein.
With the known mechanical/hydraulic clamping devices on which the present invention is based, actuation of the clamping mechanism is accomplished by displacement of the piston between a clamped position and a released position; it is pushed into the clamped position by spring elements that are arranged in a spring chamber provided between the piston and the cover, and can be brought out of it into the released position by impingement of a hydraulic medium on the pressure chamber.
One problem that repeatedly occurs in practice is that a negative pressure is created because the piston frequently moves up and down in the spring chamber, and also that cooling fluid can penetrate into the spring chamber especially if wear occurs on the seals. This can cause jamming in the closed position if the fluid that has penetrated cannot escape quickly enough from the spring chamber, and the piston thus can no longer be pushed into the upper released position.
It is therefore the object of the invention to configure a clamping device of the kind cited initially in such a way that malfunctions can be avoided.
This object is achieved, according to the present invention, in that the spring chamber is connected to the receptacle by at least one runoff duct through which the fluid that collects in the spring chamber can run off into the receptacle.
As a result of the runoff ducts provided, according to the present invention, between the spring chamber and the receptacle for the draw-in bolt, cooling fluid that enters the spring chamber can flow off into the receptacle. In addition, the creation of a negative pressure in the spring chamber is also effectively prevented, since air can also, conversely, enter the spring chamber through the runoff duct.
The runoff duct advantageously terminates at its one end in the bottom region of the spring chamber, and at its other end in the bottom region of the receptacle; the spring-chamber end should lie higher up than the receptacle end. The result of this configuration is that practically no fluid can collect in the spring chamber, and that cooling fluid cannot come into contact with components that are present in the receptacle, or with the draw-in bolt, as it runs off.
According to a preferred embodiment, provision is made for an air delivery duct, through which compressed air can be blown into the spring chamber and from there via the runoff duct on into the receptacle, to be provided in the spring chamber. By way of the blowing-out function that is provided, the receptacle can be cleared of dirt and fluid that collects therein; the piston is only imperceptibly weakened by this design as compared to known approaches such as those known, for example, from DE 197 57 430 C.